RFC 3524 Mapping Media Streams to Resource Reservation Flows April 20031. Introduction
Resource reservation protocols assign network resources to particular
flows of IP packets. When a router receives an IP packet, it applies
a filter in order to map the packet to the flow it belongs. The
router provides the IP packet with the Quality of Service (QoS)
corresponding to its flow. Routers typically use the source and the
destination IP addresses and port numbers to filter packets.
Multimedia sessions typically contain multiple media streams (e.g. an
audio stream and a video stream). In order to provide QoS for a
multimedia session it is necessary to map all the media streams to
resource reservation flows. This mapping can be performed in
different ways. Two possible ways are to map all the media streams
to a single resource reservation flow or to map every single media
stream to a different resource reservation flow. Some applications
require that the former type of mapping is performed while other
applications require the latter. It is even possible that a mixture
of both mappings is required for a particular media session. For
instance, a multimedia session with three media streams might require
that two of them are mapped into a single reservation flow while the
third media stream uses a second reservation flow.
This document defines the SDP [1] syntax needed to express how media
streams need to be mapped into reservation flows. For this purpose,
we use the SDP grouping framework [2] and define a new "semantics"
attribute called Single Reservation Flow (SRF).
1.1 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[3] and indicate requirement levels for compliant SIP
implementations.
2. SRF Semantics
We define a new "semantics" attribute within the SDP grouping
framework [2]: Single Reservation Flow (SRF).
Media lines grouped using SRF semantics SHOULD be mapped into the
same resource reservation flow. Media lines that do not belong to a
particular SRF group SHOULD NOT be mapped into the reservation flow
used for that SRF group.
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RFC 3524 Mapping Media Streams to Resource Reservation Flows April 2003
Note that an SRF group MAY consist of a single media line. In that
case, following the definition above, that media line will be mapped
into one reservation flow. That reservation flow will carry traffic
from that media line, and from no other media lines.
3. Applicability Statement
The way resource reservation works in some scenarios makes it
unnecessary to use the mechanism described in this document. Some
resource reservation protocols allow the entity generating the SDP
session description to allocate resources in both directions (i.e.,
sendrecv) for the session. In this case, the generator of the
session description can chose any particular mapping of media flows
and reservation flows.
The mechanism described in this document is useful when the remote
party needs to be involved in the resource reservation.
4. Examples
For this example, we have chosen to use SIP [4] to transport SDP
sessions and RSVP [5] to establish reservation flows. However, other
protocols or mechanisms could be used instead without affecting the
SDP syntax.
A user agent receives a SIP INVITE with the SDP below:
v=0
o=Laura 289083124 289083124 IN IP4 one.example.com
t=0 0
c=IN IP4 192.0.0.1
a=group:SRF 1 2
m=audio 30000 RTP/AVP 0
a=mid:1
m=video 30002 RTP/AVP 31
a=mid:2
This user agent uses RSVP to perform resource reservation. Since
both media streams are part of an SRF group, the user agent will
establish a single RSVP session. An RSVP session is defined by the
triple: (DestAddress, ProtocolId[, DstPort]). Table 1 shows the
parameters used to establish the RSVP session.
If the same user agent received an SDP session description with the
same media streams but without the group line, it would be free to
map the two media streams into two different RSVP sessions.
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RFC 3524 Mapping Media Streams to Resource Reservation Flows April 2003
Session Number DestAddress ProtocolId DstPort
________________________________________________
1 192.0.0.1 UDP any
Table 1: Parameters needed to establish the RSVP session
5. IANA Considerations
IANA has registered the following new "semantics" attribute for the
SDP grouping framework [2]. It has been registered in the SDP
parameters registry (http://www.iana.org/assignments/sdp-parameters)
under Semantics for the "group" SDP Attribute:
Semantics Token Reference
------------------- ----- ---------
Single Reservation flow SRF [RFC3524]
6. Security Considerations
An attacker adding group lines using the SRF semantics to an SDP
session description could force a user agent to establish a larger or
a smaller number of resource reservation flows than needed. This
could consume extra resources in the end-point or degrade the quality
of service for a particular session. It is thus STRONGLY RECOMMENDED
that integrity protection be applied to the SDP session descriptions.
For session descriptions carried in SIP, S/MIME is the natural choice
to provide such end-to-end integrity protection, as described in RFC3261 [4]. Other applications MAY use a different form of integrity
protection.
7. Acknowledgements
Jonathan Rosenberg provided useful comments about the applicability
of the mechanism described in this document.
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RFC 3524 Mapping Media Streams to Resource Reservation Flows April 200311. Full Copyright Statement
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